There is a limit to the axis load that can be permitted for highway vehicles if damage to the highway surface is to be minimized. Damage to a highway as a function of axle weight rises rapidly, once a certain load point has been passed. Government authorities closely monitor, and fine heavily, trucks that exceed these axle load limits.
The concept of an auxiliary axle that may be raised or lowered has been introduced to provide transport vehicles with a means to reduce the axle load on other axles when the critical load limit is exceeded. A disadvantage of such auxiliary axles is that due to their centrally located position on the truck body, they greatly reduce steerability when deployed with an inappropriate axle load. This is particularly true when the centrally located auxiliary axle is heavily loaded.
The loading of truck axles will vary while a truck is in motion. This can arise from a variety of conditions including wind, the gradient of hill or radius of a turn, whether the vehicle is accelerating or braking and from the shifting of cargo being carried. Thus, while a certain axle weight may be registered at a roadside scale, the actual axle load may increase over that limit, once the vehicle is in motion. These factors have given rise to systems for actually varying the load carried by an auxiliary axle, while the vehicle is in motion.
In addition to these listed factors affecting axle load the unevenness of the road surface, in terms of bumps and pot holes, can produced major transient excursions in axle loads.
A further reason for providing an auxiliary axle, and for providing a means for varying the load carried by an auxiliary axle, is that load limits may change passing from one jurisdiction to another, or when changing from a primary to a secondary road.
A problem that exists with systems designed to control the deployment and loading of auxiliary axles is that they may be bypassed by unconscientious operators who wish to carry loads that exceed proper load limits without the auxiliary axle deployed. They may choose to do this in order to improve the driveability of their vehicle in slippier conditions, to improve the ride, or on the theory that it will save tire wear.
At the same time, there are circumstances where it it would be reasonable to raise an auxiliary axle even through the maximum axle load for the remaining axles is exceeded. These include the cases where it is desired to operate a vehicle in reverse, of where it is necessary to pass the vehicle at low speed through a very tight radius turn.
Thus any sophisticated control system for the deployment of an auxiliary axle should provide both for security against evasion of the system, and for exceptions when intervention of the control system may be suspended.